Scanning probe microscopes are characterized by the nature of the local probe and its interaction with the surface. Examples of scanning probe microscopes are scanning tunneling microscopes, magnetic force microscopes, electrostatic force microscopes, scanning force microscopes and atomic force microscopes. Atomic force microscopes (AFMs) typically image a surface by applying a thin probe tip attached to the end of a flexible cantilever to the surface to be imaged. As the probe tip moves across the surface, variations in the probe tip height are detected, typically, by the tracking of a laser spot on the cantilever. Deflections of the cantilever due to height variations of the probe tip result in movement of the reflected laser spot on the optical detector. In a constant force mode application, a servo-system is used to keep movement of the reflected laser spot on the optical detector to a minimum by moving the actuator that controls the cantilever to keep the cantilever deflection approximately constant. Because the deflection of the cantilever is due to the interaction between the probe tip and the surface, keeping the deflection approximately constant is equivalent to keeping the force approximately constant. Typically, the cantilever is controlled by a piezoelectric actuator in the prior art.
Because the optical detector is responsive only to the deflection of the cantilever and not the absolute position of the tip, the typical control loop for the servo-system is an output error only loop. Therefore, the output signal from the controller of the servo-system is used as an estimate of the surface topography. This typically limits the bandwidth and accuracy of the position estimate to that of the control loop itself.